Abstract:

New post-COSTAR WFPC2 UV images of the inner bulge of M31 allow us to
investigate in more detail the composite nature of the UV flux produced by its
evolved stellar population. In particular, both luminosity function and UV
colors of the detected UV-bright objects indicate that we are likely dealing
with a minority of classical hot Post-AGB stars, whereas the relatively faint
and cool stars providing the bulk of the UV emission observed in HUT2 spectra
cannot be seen by HST as individual sources.

It is now well established that evolved stars are the responsible of the
typical UV-upturn shortward of 2000Å observed in early-type
galaxies. This conclusion comes mainly from recent space-borne UV observations
of bulge-dominated galaxies including both HST imaging (e.g., King et
al. 1992, Bertola et al. 1995) and HUT spectroscopy down to the Lyman
limit (Ferguson & Davidsen 1993, Brown et al. 1995). The most likely
candidates as the sources of the UV flux are Hot-HB, Post-EAGB and AGB-manqué
stars as well as classical P-AGB stars, whose relative contribution strictly
depends on the metallicity (and/or age) mix of the stellar population, as
discussed by Bressan, Chiosi & Fagotto (1994).

A large amount of UV observations focussed on the inner bulge of M31, the
nearest high-metallicity `early-type' subsystem characterized by a remarkable
flux rise at the shortest UV wavelengths. FOC HST images---which show
individual UV-bright stars superimposed to a fainter diffuse UV component---as
well as HUT spectra consistently indicate that the UV flux produced by this
evolved population is intrinsically composite. In fact, while a large fraction
of UV radiation can be ascribed to the presence of the variety of hot
evolutionary stages (such as Extreme-HB stars and their descendants) which
dominate the most metal-rich systems (Brown et al. 1995), in M31 a
significant contribution comes also from the shorter-lived, UV-brighter P-AGB
stars.

More precisely, our previous FOC/48 observations of M31 allowed us to detect
80 individual UV-bright stars within a field of 2323
(Bertola et al. 1995). Even if a single UV filter combination
(F150W+F130LP) was available, the existence of comparable (FOC/48 F175W)
observations by King et al. (1992) gave us the opportunity---by
combining the two sets of data---to derive a first UV color for 48 stars in
common. The subsequent comparison with proper HST c-m diagrams showed
that the detected individual sources were consistent with being
bright P-AGB stars. The main limitation of the above analysis---besides the
obvious reduced imaging capabilities of the aberrated optics of the
telescope---was the lack of a properly-spaced multi-color UV
photometry. In addition, the interpretation of the FOC data was hampered by
the redleak of the F150W+F130LP filter combination, as well as by its
possible degraded UV sensitivity, as discussed in Bertola et al.
(1995).

Figure: Luminosity Function of the 74 stars detected in the PC1 field. The
zeropoint of the magnitude scale is such that a HST magnitude m= 19.53
corresponds to a count rate of 1 count s.

Obviously, the fully restored imaging capabilities of HST at the beginning
of 1994 made this space-borne facility an even more attractive tool to achieve
our current scientific goals. They still include the detection of the
individual hot stars responsible for the UV upturn as well as the measurement
of their UV luminosity function and color(s). In order to disentangle among the
possible contributors to the UV flux, these pieces of information---coupled
with proper post-HB evolution models---are absolutely essential.

With a view to maximizing the efficiency of the optical setup for our
(post-refurbishment) carry-over observations, we moved from the FOC (whose
maximum field of view is now only 1414) to the WFPC2 which,
besides the much larger high-resolution field of PC1 (3535),
provides an improved UV performance in comparison to the progenitor WFPC-1 and
a proper filter set. Unfortunately, among the planned observations in three
consecutive filters, namely F160BW, F218W and F336W, only the last one turned
out to be of adequate S/N ratio for a meaningful analysis. However, the
combination of the WFPC2/F336W new data with the previous King's et al.
FOC/F175W data or our own FOC/F150W data, provides now a much larger wavelength
basis to derive suitable UV colors.

Figure: C-m diagram for the 37 stars detected both in our FOC/48 and
subsequent WFPC2 observations. HST magnitudes are defined in the text. A set of
isochrones for P-AGB stars of fixed (solar) metallicity (z=0.02) and different
ages (from 6.3 to 15.8Gyr, from top to bottom) are superimposed. The
nominal efficiency curve of the FOC filter combination F150W+F130LP
has been adopted for the comparison.

The observations discussed here include two consecutive UV (WFPC2 F336W) images
(2230 s exposure time in total) obtained in the context of the HST
Cycle 4. The inner nucleus of M31 was centered on the PC1 detector which
represents the highest resolution portion of the L-shaped field of the WFPC2.
Both preliminary reduction and stellar photometry were carried out by means of
standard IRAF packages. After successfully applying the gcombine task to
get a cosmic ray-free co-added image, the daofind and phot tasks
were used to identify individual point-like objects and to obtain aperture
photometry, respectively. An aperture of 3 pixels was adopted, while the
``sky'' was measured within an annulus from 10 to 15 pixels. The zeropoint
magnitude (19.53) for the filter F336W was taken from Whitmore (1995). This
implies that for our derived HST magnitude a value m= 19.53 corresponds
to a count rate of 1 count s. As already stressed, we took advantage of
our own FOC/48+F150W+F130LP pre-existing photometry of an overlapping field
to derive for as many stars as possible a UV color m-m, where
m is the HST magnitude as defined in Bertola et al. (1995). This
was made possible by the identical pattern shown by the brightest stars in both
FOC and WFPC2 images; from the coordinates of such stars one can derive the
proper transformation to identify corresponding objects (if any) from one
camera (i.e., wavelength range) to another. This technique allowed us to
obtain reliable UV colors for 37 stars.

Figure: The same as in Fig. 2, adopting for the isochrones a metallicity much
lower than solar (z=0.008).

The present investigation is confined to the portion of galaxy covered by the
PC1 detector (i.e., a field of 3535 arcsec); 74 individual
stars have been reliably (3 ) identified. Even if no strict
completeness tests have been applied, one should notice that there is no
evidence of any dichotomy in the observed LF. As a consequence, even if one can
expect---on the basis of both theoretical and observational arguments (Greggio
& Renzini 1990, Bressan et al. 1994, Brown et al. 1995)---that
more than one kind of hot component does contribute to the UV upturn, the UV
stars detected by HST likely represent a single (the brightest)
contributor to the observed flux.

The 37 stars for which we derived a FOC/WFPC2 UV color are shown in Fig. 2 and
Fig. 3 on a proper c-m diagram (m vs. m-m)
together with a set of isochrones computed for P-AGB stars of different ages
(from 6.3Gyr to 15.8Gyr) and different metallicities (z=0.02 and z=0.008).
For the theoretical computations the nominal efficiency curve of the FOC filter
combination F150W+F130LP has been adopted.

The observed good match between observed stars and such theoretical predictions
in spite of the degraded UV transmission noticed by Bertola et al. (1995)
on the basis of pre-existing IUE data, must be considered to some extent
preliminary and forces us to look further into the issue of FOC images
calibration. However, the location of the observed objects even on these
not-final diagrams, essentially assures that our HST UV images do show genuine
hot P-AGB stars. As discussed above the derived LF itself is consistent with
this conclusion. The observed spread of the points could reflect an intrinsic
spread either in metallicity or age, or both.

In summary, the present framework provided by UV space-borne investigations
(Astro-1, Astro-2 and HST) indicates that, whereas the bulk of the UV emission
produced by (metal-rich) early-type galaxies comes from stars relatively cool
(T20,000--23,000 K), a significant contribution can also come---as in the
case of the inner bulge of M31---from classical hotter P-AGB stars. Since the
latter stars are by far the brightest in the UV, they are the only objects
detected by HST as individual sources.